C - 调用read（）函数Linux时的奇怪的termios行为

Question

通过串口读取ELM327芯片时遇到问题。 这是我的代码

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <string.h
#include <sys/ioctl.h>

#define DEVICE "/dev/ttyUSB0"
#define SPEED B38400 

int main()
{
    struct termios old_stdio; //save the current port settings
    int tty_fd; //file descriptor for serial port
    int retval, res, n, res2, read1, wri;
    char buf[255];
    char buf2[255]; 

    tty_fd = open(DEVICE, O_RDWR | O_NOCTTY | O_NDELAY );
    if(tty_fd < 0)
    {
        perror(DEVICE);
        exit(-1);
    }
    printf("Init 1 complete.\n");

    tcflush(tty_fd, TCIOFLUSH);

    fcntl(tty_fd, F_SETFL, 0);


    if(tcgetattr(tty_fd, &old_stdio) != 0)
    {
        perror(DEVICE);
        exit(-1);
    }
    printf("Init 2 complete.\n");

    struct termios newtio;


    if(tcgetattr(tty_fd, &newtio) != 0)
    {
        perror(DEVICE);
        exit(-1);
    }

    cfsetospeed(&newtio, B38400);
    cfsetispeed(&newtio, B38400);

    printf("Init 3 complete.\n");
    newtio.c_cflag |= CLOCAL | CREAD | HUPCL;
    newtio.c_iflag |= ICRNL | IGNPAR;
    newtio.c_lflag &= !(ICANON | ISIG);
    newtio.c_lflag |=  ECHOK | ECHOE | ECHOKE;
    newtio.c_oflag |= ONLCR;

    newtio.c_cc[VTIME] = 0;
    newtio.c_cc[VMIN] = 0;


    if(tcsetattr(tty_fd, TCSANOW, &newtio) != 0)
    {
        perror(DEVICE);
        exit(-1);
    }
    printf("Init 4 complete.\n");


    for(n = 55; n > 0; n--)
    {
                printf("Please enter a command: ");
                (void)fgets(buf2, 255, stdin);
                printf("Ok.input was %s ",&buf2); 
                (void)write(tty_fd, buf2, strlen(buf2));
                (void)write(tty_fd, "\r", 1);
                printf("Ok. Waiting for reply\n");
                res = read(tty_fd, &buf, 255);       
                printf("Read:%d %s \n",res, &buf);              
    }

    //restore the original port settings
    tcsetattr(tty_fd, TCSANOW, &old_stdio); 

        close(tty_fd);

    return EXIT_SUCCESS; //return all good
}

我可以写信给设备，但写完atz之后，阅读功能并没有读取任何东西。但在再次输入\n之后，它会在终端打印输出。

Init 1 complete.
Init 2 complete.
Init 3 complete.
Init 4 complete.
Please enter a command: atz
Ok.input was atz
Ok. Waiting for reply
Read:0  
Please enter a command: 
Ok.input was 
Ok. Waiting for reply
Read:20 atz


ELM327 v1.5

>� 
Please enter a command: 

任何人都知道为什么会发生这种情况？